(1) Field of the Invention
The present invention relates to a color image processing apparatus, in particular, relates to a color conversion device in a color image processing apparatus for obtaining the identical color reproducibility, in color scanners, color cameras, color hard copy devices, color image input devices, display devices, output devices and the like in which high-speed color adjustment and color correction are necessary.
(2) Description of the Prior Art
As the color conversion method in the field of color printing and color hard copy, there has been conventionally proposed a three-dimensional interpolation using a look-up table, which is disclosed in, for example, Japanese Patent Application Laid-open Sho 63 No. 162248.
As shown in FIG. 1A, this is a method for performing the color conversion, comprising steps of dividing the three-dimensional color space into a plurality of unit cube, setting a color correction value obtained by a calculation in advance to each lattice point of the cubic pattern, selecting a unit cube containing an input color, and using an output value at a plurality of apexes of the unit cube. In this example, it is characterized in that, as shown in FIG. 1B, the values of eight apexes of the unit cube are used for the interpolation processing, in which a correction value is obtained by designating an apex opposite to a point of a correction value to be determined and a volume in a space area of a rectangular parallelepiped formed at the input point as a weight coefficients at a point of the correction value to be determined.
With the conventional color conversion method as described above, however, as the input color space is divided more fine, that is, as the distance between the lattice points becomes smaller, the accuracy of interpolation becomes high, but the lattice points increase. Hence, the memory capacity for storing the color correction value set at the lattice points increases, resulting in increase of processing time for the calculation and complexity in the hardware structure. Moreover, it is time-consuming to obtain the correction value itself to be stored.
Furthermore, it is characterized in that the accuracy necessary for each axis in the normal three-dimensional color space is nonuniform, hence when the interpolation method is applied for the color conversion, it is preferable that the bit number of each axis in the table memory used for the interpolation be not uniform. If the total bit number is constant, high-quality interpolation of the color information becomes possible which suits the human's visual property without increasing the memory capacity, by taking nonuniform bit structure. However, with the conventional technique, since the basic unit of interpolation is a unit cube within the three-dimensional color space made by the input signal, there is a problem in that the interpolation taking the human's visual property into account cannot be performed.